01 - Preheat oven to 350°F. Thoroughly grease and flour a 10 to 12-cup bundt pan.
02 - In a medium bowl, whisk together flour, poppy seeds, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, beat softened butter and sugar until light and fluffy, about 3 to 4 minutes.
04 - Add eggs one at a time, beating well after each. Mix in lemon zest, lemon juice, and vanilla extract.
05 - At low speed, alternately add the flour mixture and the combined sour cream and milk, beginning and ending with the flour mixture. Mix just until combined to avoid overmixing.
06 - Pour batter evenly into prepared pan, smoothing the top. Bake for 45 to 50 minutes, or until a toothpick inserted in the center comes out clean.
07 - Let cake cool in the pan for 15 minutes before inverting onto a wire rack to cool completely.
08 - Whisk powdered sugar, lemon juice, heavy cream, and lemon zest until smooth and pourable. Adjust consistency with more cream or sugar if necessary.
09 - Drizzle glaze evenly over cooled cake. Allow to set before slicing and serving.